The present invention relates to the art of induction heating and, more particularly, to an inductor and inductor assembly for heating bearing surfaces of a crankshaft and methods of making such an inductor.
The present invention finds particular utility in conjunction with the inductive heating of the bearing surface of a crankshaft journal pin and will be described with particular reference thereto. At the same time, however, it will be appreciated that the invention is applicable to the induction heating of the bearing surface of the crankshaft journal as well as annular bearing surfaces of other work-pieces.
In conjunction with the induction hardening of automotive crankshaft bearing surfaces, there is an increasing demand by the automotive industry for selectively hardening crankshaft pin and main bearing surfaces. Such selective hardening has been motivated by the demand in the automotive sector for higher output engines which impose increased requirements on bearing loads and structural strength. Moreover, newer engine designs are being reduced in size while maintaining the same or an increased output capability which requires operation of the engines at higher torque and stress levels. At the same time, the reduction in size results in a substantially reduced axial length of the crankshaft pin and main bearings.
It is of course known to inductively heat the bearing surface of a crankshaft journal pin through the use of an arcuate inductor of fabricated tubular construction positioned relative to the bearing surface as the latter orbits during a heat treating operation. In order to obtain efficient operation and high quality heat treating of crankshaft bearing surfaces, the active element of the inductor must be dimensionally accurate and consistently positioned accurately relative to the bearing surface so as to consistently duplicate the desired hardening results during repeated use of the induction heating apparatus. Even if the active inductor element is supported for tweaking adjustment relative to the bearing surface to be inductively heated, such adjustment impacts consistent quality assurance since it involves a manual operation and accordingly can vary and change from one equipment setup to another.
As shown in U.S. Pat. No. 3,188,440 to Wokas, for example, it has been the practice heretofore to manufacture the active inductor elements of an inductor for inductively heating crankshaft bearing surfaces from fabricated or formed tubing, and there are a number of problems and disadvantages attendant to this practice. To begin with, formed or fabricated tubing has limited dimensional accuracy due, at least in part, to the bending and/or cutting and/or brazing operations of which there are a considerable number in connection with fabricating the active elements of an inductor such as that disclosed in the Wokas patent. Further, brazed joints between tubing sections lower efficiency and provide areas of stress concentration which lead to early failures, and the latter is dependent to a considerable extent on the skill of the person doing the brazing, whereby a high degree of brazing skill is required in an effort to minimize such failures. Still further, brazed joint interfaces are of higher resistivity resulting in thermally induced stress and higher thermal loss during operation of the inductor. A further disadvantage of fabricated tubular inductors resides in the fact that the internal cooling path includes a number of sharper angular directional changes which can impede the flow of cooling water therethrough, thus reducing the desired thermal maximum transfer of heat to the cooling water and result in undesirable surface/interface conditions. Still further, to meet the new requirements for inductively heating axially shorter bearing surfaces, space availability is a factor which compromises all of the foregoing problems. Still further, fabricated tubing limits the optimum geometry of the active inductor element as well as the optimum geometry of the internal cooling water passages and, as a result, both cooling and durability of the inductor are compromised because the thermal energy generated within the inductor cannot be optimally effectively transferred to the cooling water. It is known, as shown in U.S. Pat. No. 4,535,211 to Carter, to construct a "single shot inductor" for inductively heating axle shafts from a single block of copper so as to eliminate brazed joints between adjacent inductor sections and obtain improved accuracy with respect to dimensional tolerances. The physical structure of a "single shot inductor", however, is totally different from that of an inductor for inductively heating a narrow crankshaft bearing surface and accordingly, as will become apparent hereinafter, the manufacturing techniques required to produce an inductor for inductively heating a crankshaft bearing surface in accordance with the present invention vary considerably from that for producing a "single shot inductor".
A further disadvantage with crankshaft bearing surface induction heating assemblies heretofore provided, such as that of Wokas, is that the active inductor element is not supported with sufficient structural integrity to assure consistency with respect to maintaining the proper position thereof relative to the moving bearing surface, especially with respect to the pin bearing which is orbiting during the induction heating operation. While Wokas provides side plates supporting guide fingers for positioning the active inductor element relative to the bearing surface, the inductor element is supported in suspension relative to the bearing surface by the inductor and coolant line leads connected thereto. The latter are between and spaced from the side plates and, accordingly, the active inductor element can move both axially and radially relative to the guide fingers and thus the bearing surface, and such relative displacement capability is a detriment to maintaining proper positioning of the inductor element relative to the bearing surface. Moreover, the spaced relationship between the inductor leads and side plates in the Wokas inductor assembly would promote stray heating of the side plates which would reduce the efficiency with respect to inductively heating of the bearing surface through the active inductor element. Furthermore, this heating causes mechanical movement which causes changes in inductor location accuracy. These changes can be inconsistent transitional mechanical movements plus sequentially additive changes, all of which equate to thermal ratcheting.